Effects of aging on shear bond strength to dentin and mechanical properties of restorative glass ionomer cements

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Abstract

Objectives

To evaluate the shear bond strength to dentin (SBS), flexural strength (FS), and diametral tensile strength (DTS) of four restorative glass ionomer cements after aging in artificial saliva.

Materials and methods

For SBS testing, sound molars were ground to flat occlusal dentin and fixed with acrylic resin. Teeth were randomly divided into four groups along with the following materials: Ketac Nano, Ketac Molar, Fuji II LC and Equia. Glass ionomer cylinders were bonded to each dentin surface. For testing the mechanical properties, FS bars (25 mm × 2 mm x 2 mm) and DTS discs (4 mm × 2 mm) were fabricated. All specimens were stored in artificial saliva (37 °C) and tested after 24 h and 6 months. Data was analyzed with two-way ANOVA and post-hoc Tukey's tests (α = 5%).

Results

The SBS results were statistically similar among the four glass ionomer cements (p > 0.05). No significant differences in SBS after 24-h and 6-month storage were found, except for in the Ketac Nano group, where a significant decrease in SBS was found after aging (p < 0.05). All materials had a significant increase in FS after aging. The DTS values did not change significantly after aging, with the exception of the Fuji II LC group. Overall, the Fuji II LC group showed significantly higher DTS and FS compared to the other materials.

Conclusions

Aging for 6 months did not affect the SBS of restorative glass ionomer cements, except in the case of Ketac Nano. All glass ionomer cements showed increased FS over time. The DTS remained unchanged – except for the Fuji II LC, which experienced increased DTS.

Introduction

Glass ionomers have been regarded as a “smart material” due to their ability to release fluoride after setting. Studies have suggested that glass ionomer cements (GICs) can be recharged with fluoride when exposed to the oral environment [1] and that the anti-cariogenic biofilm activity can be closely correlated with the fluoride release rate during biofilm formation [2]. Current literature also supports the use of glass ionomer liners for indirect pulp capping procedures, as they can aid dentin remineralization and arrest caries [3]. Additionally, glass ionomer cements exhibit minimal volumetric changes during the setting reaction. This is beneficial when compared to resin-based composites, as the latter undergo polymerization shrinkage during setting – contributing to potential post-operative sensitivity and marginal breakdown over time [4]. Although GICs do not provide bond strength values comparable to dental adhesives, their characteristics provide excellent interfacial sealing and they are also less affected by regional variances of the substrate [[5], [6], [7]].

A previous study speculated that the bonded interface between a GIC and dentin would not be greatly influenced by 24 h of water storage [8]. More recently, one study investigated the effect of 6- and 12-month water storage on the SBS of resin-modified (RM)GICs bonded to dentin, with and without chlorhexidine pre-treatment. The results demonstrated a significant increase in SBS values for the control group after aging [9]. This important finding clearly requires further investigation, as the bond durability of glass ionomer restorative cements to tooth structure over time remains largely untested. In contrast, resin-dentin interfaces present a significant decrease in bond strengths after aging. Multiple studies have demonstrated the degradation of resin-dentin bonds on resin composite restorations over time [10,11].

Most in vitro studies are mainly short term and the effect of environment and time has not been extensively studied for glass ionomers. One study demonstrated that the mechanical properties of glass ionomers were not affected by storage in water at 37 °C over a period of 1 year [12]. There is insufficient evidence to assess the impact of aging on the bond strengths and mechanical properties of glass ionomer cements. The lack of conclusive data propels one to investigate the influence of long-term storage on the bond strength and mechanical properties of restorative glass ionomer cements. Thus, the aim of the present study was to evaluate the effects of artificial aging on flexural strength, diametral tensile strength, and shear bond strength of restorative glass ionomer cements to dentin. The null hypotheses to be tested are the following: 1. Artificial saliva aging of restorative glass ionomer cements does not affect shear bond strength to dentin and 2. Artificial saliva aging of restorative glass ionomer cements does not affect the flexural strength and diametral tensile strength of restorative glass ionomer cements.

Section snippets

Materials and Methods

Four restorative glass ionomer cements were used in this study: Ketac Nano (3 M/ESPE), Ketac Molar (3 M/ESPE), Fuji II LC (GC America Corp.), and Equia (GC America Corp). The composition of each glass ionomer cement, their corresponding dentin conditioners, and application mode are described in Table 1.

Shear bond strength to dentin

The shear bond strength to dentin results (Table 2) showed a statistically significant difference among materials after allowing for effects of 24-h and 6-month storage time (p = 0.008). All pairwise multiple comparisons (Tuckey method) showed no significant changes in SBS values after 6-month artificial saliva aging, with the exception of the Ketac Nano specimens which showed a significant decrease in shear bond strength (p = 0.0046). The mode of failure analysis results (Fig. 1) demonstrated

Discussion

The shear bond strength results led us to reject the first null hypothesis. While 6-month aging in artificial saliva did not affect the bond strength to dentin for most materials, Ketac Nano specimens presented a significant decrease in SBS values after storage. It is likely that the Ketac Nano chemical composition may have impacted the results [15,16] for two main reasons. First, it requires the application of a light curable primer, which is composed mostly of water (40–50%) and

Conclusions

In conclusion, this research demonstrates that the shear bond strength to dentin exhibited stability over time for most of the materials tested. There was one exception, Ketac Nano, which showed a decrease in shear bond strength over time. Flexural strength values exhibited a significant increase after aging for all restorative glass ionomer cements evaluated. In contrast, diametral tensile strengths, for most of the materials tested, were not affected by aging in artificial saliva. Among all

Acknowledgements

This work was supported by start-up funds from The University of British Columbia’s Faculty of Dentistry. The authors thank Angela Tether for proof reading and editing the article.

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